1. Field of the Invention
The present invention relates to optical pickup devices forming a microspot on an information recording medium to optically densely record and reproduce information.
2. Description of the Background Art
In recent years, large capacities of data such as digital still pictures and moving pictures are increasingly used as multimedia advances. In general, such data is stored in a large-capacity recording medium such as optical discs and randomly accessed and reproduced as required. An optical disc is randomly accessible and has a recording density higher than magnetic recording media such as floppy discs. Furthermore, magneto-optical discs, which are rewritable, can be used as a recording medium as above. Most of such magneto-optical discs have an information recording layer with convex and concave portions referred to as lands and grooves, respectively, used as a tracking guide.
Such recording media as optical discs and magneto-optical discs are increasingly adapted to record data densely to be able to record larger capacities of data. For example a medium would have a track pitch reduced to increase a linear density in the direction of the track or have a minimal mark length reduced to enhance its recording density in its tangential direction to record data densely.
Furthermore, even if a highly efficient data compression system such as MPEG2 is used to record and reproduce moving pictures of high quality, the system is still required to transfer data at as high a rate as 10 Mbps to 20 Mbps. To record such a moving picture at real-time while an impact or the like has caused a servo to be displaced, such system must record it at at least 1.5 times the above data transfer rate. Accordingly the optical disc of interest must be rotated faster to increase linear velocity. A higher linear velocity entails a higher recording power and this requires that light availability, i.e., an optical output from an objective lens that originates from an optical output from a light source, be maximized.
Furthermore, for potable applications, recording media are increasingly adapted to have smaller sizes and optical pickup devices are accordingly required to have smaller sizes (in weight and volume).
Thus to accommodate a recording medium of large capacity and high transfer rate an optical pickup device is required to minimize in size a spot converged and thus formed on the recording medium and ensure a recording power of high output. Furthermore, as recording media are miniaturized optical systems are also required to be miniaturized.
Such optical pickup devices in general use semiconductor laser as their optical source. As shown in FIG. 14, a semiconductor laser 101 currently put to use varies in angle of divergence in the y direction in the x-y plane parallel to a surface 101c joining laser chips 101a and 101b together and in the z direction in the x-z plane perpendicular to joint surface 101c. An AlGaInP-based semiconductor laser of approximately 650 nm in wavelength provides an angle of divergence of approximately eight degrees in the y direction and an angle of divergence of approximately 24 degrees in the z direction, as represented in full width half maximum, and a region having a uniform optical intensity in a cross section in the y-z plane perpendicular to an optical axis of a light beam has an elliptic pattern with its shorter and longer axes corresponding to the y and z directions, respectively. If ellipticity is defined by a ratio of a diameter in the longer axis""s direction to that in the shorter axis""s direction, then the AlGaInP-based semiconductor laser would have an ellipticity of three.
Furthermore a GaN-based semiconductor laser of approximately 400 nm in wavelength that is currently being developed would have a further increased ellipticity of approximately four.
Furthermore, while a light emitted from semiconductor laser 101 in general polarizes in a direction parallel to joint surface 101c or the y direction, a light emitted from semiconductor laser 101 for example of approximately 635 nm in wavelength can polarize in a direction perpendicular to joint surface 101c or the z direction.
Recording a larger capacity of data on a recording medium can be achieved simply by minimizing the area of a spot converged on the recording medium and optimizing the recording medium""s track pitch and shortest mark length to match the shape of the converged spot. A converged spot has its area minimized when it is a round spot with its diameter corresponding to diffraction limited. Two techniques can be used to obtain such round, converged spot from a light beam having an elliptic cross section.
The first technique uses beam shaping means such as a shaping prism to allow a light beam incident on an objective lens to have an isotropic intensity distribution. Reference will now be made to FIG. 15 to describe the shaping prism""s operation. Shaping prism 105 has a receiving side, with the FIG. 14 xyz coordinate system considered, and an outputting side, with an xxe2x80x2yxe2x80x2z coordinate system considered. The xxe2x80x2 axis is adapted to be parallel to an optical axis of a light beam emerging from shaping prism 105. One coordinate system corresponds to the other coordinate system with the x and y axes rotated around the z axis by a predetermined angle.
When shaping prism 105, in the form of a wedge, receives a collimated light beam having a diameter Din (Y) in the y direction and a diameter Din (Z) in the z direction, incident on a plane of incidence 105a at an angle of xcex81, the light beam is refracted at an angle of refraction xcex82. If shaping prism 105 is formed of a material having an index of refraction n then the following relationship is established:
sin(xcex81)=nxc3x97sin(xcex82).
The refracted light beam is incident on a plane of emergence 105b perpendicularly and it is thus not refracted at the plane of emergence 105b and emerges in the form of a collimated beam having a diameter Dout (Yxe2x80x2).
Thus, diameter Din (Y) in the y direction is increased by shaping prism 105 by Dout (Yxe2x80x2).
In contrast, diameter Din (Z) in the z direction is not shaped by shaping prism 105 and thus emerges from shaping prism 105 as it is, i.e., Dout (Z)=Din (Z).
Furthermore the beam""s direction of emergence is polarized relative to its direction of incidence in the x-y plane by a predetermined angle.
Herein the ratio of Dout to Din defines shaping-ratio. For example an elliptic cross section of an ellipticity of three can be converted to a round cross section by setting the shape and index of refraction of shaping prism 105 and the angle of incidence xcex81 and the angle of refraction xcex82 to set a shaping ratio equal to the ellipticity of three to increase its diameter in the shorter axis""s direction three times.
More specifically, a light beam having a wavelength of 655 nm and shaping prism 105 formed of BK7 result in an index of refraction of 1.51389. As such, with a xcex81 of 75.13 degrees and a xcex82 of 39.68 degrees, shaping prism 105 having the plane of incidence 105a and the plane of emergence 105b forming an angle of 39.68 degrees can provide the shaping ratio of three.
The second technique uses only a portion of a light beam having an elliptic cross section that is close to the optical axis and has an isotropic intensity distribution. This can be implemented by increasing a focal length of a collimator lens to reduce an effective NA (i.e., a radius of an effective aperture of an objective lens that is divided by a focal length of a collimator lens) to 0.1 or therebelow.
Optical pickup devices corresponding to the first and second techniques are configured as will be described below:
As a first conventional example, FIG. 16 shows an optical pickup device employing a shaping prism corresponding to the first technique to shape a beam. In the figure, semiconductor laser 101 emits an anisotropic light beam which then proceeds as a P polarization via a diffraction element 102 and is then incident on a collimator lens 104 in the form of a divergent beam of light having an elliptic cross section with its longer axis extending in the z direction. The divergent beam is collimated by collimator lens 104 and thus provided as a collimated light beam still having an elliptic cross section. The collimated light beam is incident on shaping prism 105, with its optical axis tilted, and at the prism""s plane of incidence 105a it is rounded and refracted and thus incident on a polarized-beam splitter 103 in the form of a collimated light beam having a round cross section of an ellipticity of one.
The collimated light beam round in cross section is transmitted through polarized-beam splitter 103 and then converged by objective lens 106 onto an optical disc (magneto-optical disc) 107. Optical disc 107 provides a reflection of the light converged thereon which has been magneto-optically affected and thus has a plane of polarization rotated to provide the reflection in the form of a light beam containing an S polarization component. This light beam proceeds again via objective lens 106 towards semiconductor laser 101 and is thus incident on polarized-beam splitter 103.
Polarized-beam splitter 103 transmits and reflects light reflected from optical disc 107. The light transmitted through polarized-beam splitter 103 is passed via shaping prism 105 and collimator lens 104 and thus incident on diffraction element 102 having a surface 102a with a diffraction grating 121 formed thereon for generating a servo signal. The light beam incident on diffraction element 102 is thus partially diffracted by diffraction grating 121 in the y direction and thus incident on a photodetector 108, which in turn provides an output from which a focus error signal (FES) and a tracking error signal (TES) are detected.
Diffraction element 102 has a surface 102b provided with a diffraction grating 120 for generating three beams. If a diffracted light from diffraction grating 121 passes through diffraction grating 120, it is accordingly diffracted and its quantity of light is accordingly reduced. This causes a servo signal to be offset. As such, diffraction element 102 is adapted to have a sufficient thickness to prevent a diffracted light from diffraction grating 121 from being incident on diffraction grating 120.
Diffraction element 102 is adhered to and thus fixed on an upper surface of a package accommodating semiconductor laser 101 and photodetector 108, to form an integrated unit referred to as a hologram laser 112.
Furthermore a light reflected from optical disc 107 that is reflected by polarized-beam splitter 103 is separated by Wollaston prism 109 into a P polarization component and an S polarization component, which in turn have their optical paths bent by a prism mirror 113 and their respective spot sizes adjusted by a convex lens 114 and thus converge in two spots on a light receiving surface of a photodetector 111. The two signals can be operated on to provide their differential signal to detect a high-quality, magneto-optical signal having cancelled a noise component such as a variation in the reflectance of optical disc 7.
Wollaston prism 109 is formed by joining together two wedge-shaped members formed of a birefringent material such as crystal and lithium niobate and having their respective, crystallographic optical axes in directions set at +45 degrees and xe2x88x9245 degrees, respectively, to the P polarization and thus orthogonal to each other in a plane orthogonal to an optical axis.
Furthermore, as shown in FIG. 16, shaping prism 105, polarized-beam splitter 103, Wollaston prism 109 and prism mirror 113 can be integrated. As such, such components can be can be arranged in a smaller space and also readily adhered and fixed to their base (not shown).
As a second conventional example, FIG. 17 shows an optical pickup device employing the second technique, using only a portion of light in a vicinity of an optical axis. In the figure, semiconductor laser 101 emits an anisotropic light beam, which in turn proceeds as a P polarization via diffraction element 102 and polarized-beam splitter 103 and is then incident on collimator lens 104 in the form of a divergent light beam having an elliptic cross section with its longer axis extending in the z direction and it is then converged by objective lens 106 onto optical disc (magneto-optical disc) 107.
Optical disc 107 provides a reflection of the light converged thereon, which then proceeds again via objective lens 106 and collimator lens 104 towards semiconductor laser 101 and is thus incident on polarized-beam splitter 103.
Polarized-beam splitter 103 transmits and reflects the light reflected from optical disc 107. A light transmitted through polarized-beam splitter 103 is incident on diffraction element 102 having surface 102b with diffraction grating 121 formed thereon and the light is thus partially diffracted by diffraction grating 121 in the y direction and thus incident on photodetector 108 providing an output from which an focus error signal and an tracking error signal are detected.
Diffraction element 102 is adhered to and thus fixed on an upper surface of a package accommodating semiconductor laser 101 and photodetector 108, to form an integrated unit referred to as hologram laser 112.
On the other hand, a light reflected from optical disc 107 and then by polarized-beam splitter 103 is separated by Wollaston prism 109 into a P polarization component and an S polarization component, which in turn have their spot sizes adjusted by convex lens 110 and are thus converged in two spots on a light receiving surface of photodetector 111.
Since polarized-beam splitter 103 can be arranged in a divergent flux between semiconductor laser 101 and collimator lens 104, objective lens 106 and collimator lens 104 can be arranged adjacent to each other to provide a miniaturized optical system. Furthermore, while the first conventional example shown in FIG. 16 requires convex lens 114 for converging light on photodetector 111, the second conventional example can use collimator lens 104 also serving to converge light on photodetector 111 to provide a reduced optical path length and hence a further miniaturized optical system.
In the first conventional example, however, while a converged spot has a round shape, in order to ensure sufficient light availability, collimator lens 104 needs to have an effective NA increased to approximately 0.3 and a reduced focal length. Accordingly, collimator lens 104 and semiconductor laser 101 are arranged disadvantageously closer to each other and polarized-beam splitter 103 can thus not be arranged in a converged flux between collimator lens 104 and semiconductor laser 101. In particular, with hologram laser 112 used, diffraction grating 102 arranged in a converged flux renders it further difficult to arrange polarized-beam splitter 103.
Since polarized-beam splitter 103 and other components are arranged in a collimated flux, such components need to have an effective aperture covering the size of the collimated flux and thus be increased in size, and convex lens 114 is also required for converging light on photodetector 111. This disadvantageously increases the size of the optical system of interest.
In the second conventional example, light availability is extremely reduced since to obtain a round converged spot a collimator lens has an effective NA reduced to 0.1 or therebelow to use only a portion of a flux obtained in a vicinity of an optical axis. This example can be adopted if it is solely used for reproducing information and there is sufficient margin in light availability, although its low light availability must be compensated for by accordingly, excessively increasing the semiconductor laser""s output and its power consumption can thus not be reduced.
If the collimator lens has its numerical aperture increased to approximately 0.2 to ensure sufficient light availability, an elliptic converged spot and hence a poor focus would result and the example can thus not accommodate increasing the density of the recording medium of interest. As such in the second conventional example there is a trade-off relationship between obtaining a round converged spot and obtaining increased light availability and they can hardly be obtained simultaneously.
The present invention has been made to overcome such disadvantage as above, ensuring sufficient light availability while providing a converged spot sufficiently reduced in diameter and a device reduced in size.
In accordance with the present invention an optical pickup device includes a semiconductor laser; a collimator lens converting to a collimated flux a divergent flux emitted from the semiconductor laser; beam shaping means converting a ratio between shorter and longer diameters of an elliptic cross section of the collimated flux formed by the collimator lens; and an objective lens converging on an information recording medium the flux output from the beam shaping means, and receiving a flux reflected from the information recording medium; wherein the beam shaping means shapes a beam to allow the flux output therefrom to have an elliptic cross section having a ratio between shorter and longer diameters of no more than two.
Thus a light beam incident on the objective lens can have an improved ellipticity to allow a converged spot to effectively have an area stopped down, reduced to be substantially equal to an area of a beam spot shaped completely round in cross section.
The present invention preferably includes light separation means separating the flux reflected from the information recording medium in a direction of the semiconductor laser and in a direction different from the direction of the semiconductor laser, and a diffraction element diffracting the flux reflected from the information recording medium and directing the diffracted flux to a photodetector, wherein the light separation means and the diffraction element are arranged in an optical path extending between the collimator lens and the semiconductor laser.
In present invention, preferably the beam shaping means increases the shorter diameter of the elliptic cross section to shape a beam.
In the present invention, preferably the beam shaping means reduces the longer diameter of the elliptic cross section to shape a beam.
In the present invention, preferably the diffraction element and the light separation means are spaced by a set range of 0.5 mm to 2.0 mm and the light separation means and the collimator lens are spaced by a set range of 0.5 mm to 2.5 mm.
In the present invention, preferably the beam shaping means diffracts a beam at least twice to shape the beam.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.